1. Field of the Invention
The present invention relates to an image reading apparatus for use in an electronic file, a facsimile apparatus or the like, and more particularly to such image reading apparatus capable of providing satisfactory image signals by correcting so-called shading or fluctuation in the image signals obtained by reading an image.
2. Description of the Prior Art
It has recently become well known to read an original document with a solid-state image sensor such as a charge-coupled device and to apply various digital processing to a thus obtained analog electric signal after analog-to-digital conversion, for the purpose of storage in an optical or magnetic disk, image formation on a printer, or transmission to a distant location. In this connection, recent advances in digital technology have enabled electrical correction of the so-called shading phenomenon, or fluctuation of the output signals of a scan line of the charge-coupled device resulting from nonuniformity in the intensity of the illuminating fluorescent lamp, in the luminance distribution in the optical system and in the sensitivity of the charge-coupled device, and such correction has become an important factor in the reproduction of intermediate tones in the image. Such digital correction is generally achieved by storing corrective values, obtained by reading for example a white reference board, in a random access memory, and supplying said corrective values to a processing circuit together with digital signals which are obtained by reading an original and are to be corrected, thereby effecting corrective processing.
However, higher frequencies are more and more required in the processing due to the ever increasing frequency of the digital signals to be corrected, so that it is often not possible to provide a correspondingly fast data processing, or is necessary to use an expensive processing circuit in order to meet the requirement of complex data processing.